The reduction of crop growth and yield in dry areas is largely due to stomatal closure in response to dry soil, which decreases photosynthesis. However, the mechanism that causes stomatal closure in a drying soil is a controversial issue. Experienced and respected plant physiologists around the world have different views about the primary sensor of soil water shortage in plants. The goal of this review is to present a chronological synthesis about the evidence of the possible candidates for the mechanism by which plants sense a drying soil. Hydraulic signals in the leaves as the mechanism that causes stomatal closure dominated the view on how plants sense a drying soil during the 70’s and the early 80’s. In the middle 80’s, studies suggested that stomatal conductance is better correlated with soil and root water status than with leaf water status. Thus, chemical signals produced in the roots dominated the view on how plants sense a drying soil during the late 80’s and early 90’s. During the second half of the 90’s, however, studies provided evidence that hydraulic signals in the leaves are still better candidates for the mechanism by which plants sense a drying soil. After more than 60 years of studies in plant-water relations, the question raised in the title still has no unanimous answer. This controversial issue is a good research rationale for the current generation of plant physiologists.
soil water; plant water; water stress; stomatal conductance